Standardise labels

The label is set from the --label passed to bilby_pipe_analysis, which is in turn set by these lines of code. The meta_label there was introduced to provide arbitrary extra information, e.g. which row of the injection file/GPS-time-file it referred to. For event data, there is not meta-data and so it is coming out as __. We need this to be standardised to help in programatically discovering results.